1. Field of the Invention
The present invention generally relates to a mobile communications system and, more particularly, to a method apparatus for determining handover by considering channel status of a terminal in a mobile communications system.
2. Description of the Related Art
In a mobile communications system, when a terminal migrates to a boundary region between cells, the terminal performs handover.
To perform the handover, the terminal determines whether a handover condition is satisfied by periodically measuring signals of a neighbor Base Station (BS). When the handover condition is satisfied, the terminal generates and transmits a measurement report message to its serving BS. The serving BS determines the handover of the terminal by referring to the measurement report message, and performs a procedure to hand the terminal over to a target BS. The handover condition signifies that a difference between a signal strength of the serving BS received at the terminal and a signal strength of the neighbor BS satisfies a hysteresis value during a Time To Trigger (TTT). The TTT and the hysteresis value are provided from the mobile communications system to the terminal and managed per BS.
FIG. 1 illustrates a handover process in a general mobile communications system. In FIG. 1, a User Equipment (UE) 100 transmits and receives packet data to and from a source Evolved Node B (eNB) 104 and a MME/S-GW 106 in step 110, and measures Reference Signal Receive Power (RSRP) values of the source eNB 104 and a neighbor eNB in step 112. When the RSRP value of the source eNB 104 is lower than a threshold, the UE 100 measures the RSRP value of the neighbor eNB 102, compares the measured RSRP value with the RSRP value of the source eNB 104, determines whether the RSRP difference is greater than the hysteresis value, and determines whether this state is maintained for the TTT. When this state is sustained for the TTT, the UE 100 generates and sends the measurement report message to the source eNB 104 in step 114.The source eNB 104 determines the neighbor eNB 102 as a handover target eNB 102 of the UE 100 in step 116, sends a handover request message to the target eNB 102 in step 118, determines a resource of the target eNB 102 in step 120, and receives a handover request response message in step 122. The source eNB 104 then sends an Radio Resource Control (RRC) connection reconfiguration message instructing the handover to the UE 100 in step 124.
The UE 100 hands over to the target eNB 102 and transmits and receives packet data via the target eNB 102.
As explained above, the conventional mobile communications system determines whether to hand over depending on whether the RSRP value difference of the source eNB and the neighbor eNB meets the hysteresis value during the TTT. However, this method cannot select an optimum handover time and may thus cause a ping-pong situation in which the UE frequently hands over, which can delay the handover and waste radio resources, incur additional delay due to retransmission, and can lose data. For example, if the UE is travelling slowly and the TTT and the hysteresis value are small, handovers can frequently occur. When the UE travels fast and the TTT and the hysteresis value are great, the handover may not proceed at the necessary time.